Heavy Lift Specialist
Sep 282012
 

I conclude this serie of 3 articles on multiple crane lifts with the erection of a pressure vessel  of 520 tons by means of two main lift cranes and one tailcrane.

The  pressure vessel which needs to be erected is shown in Fig. 1 .

We will make use of the technics which we learned in the previously discussed topics and  which are summarized below:

  • Decrease of tailload due to offset of tailing lug
  • Decrease of tailload due to position of main lifting trunions close to the CoG of the column

Enormous Crane
Due to the enormous weight and lenght of  the column it would require a very large crane to erect it from horizontal into vertical position. An alternative method would be the erection of this column by means of a mast gantry system (or Gin Poles). The use of such a lifting gantry is though  time consuming and  occupies a large area of the construction site due to guywires which are in many cases needed.  Guyless lifting gantries are available as well ,  but they still require a rather big crane to assist in the erection of the gantry itself. Another disadvantage of the mast gantry system is the high point load underneath the legs of the gantry, which must be absorbed by specially constructed foundations.

Depending on the location of the jobsite, erection by means of two main lift cranes and one tailcrane, offers in general a far more efficient and faster erection method than the use of lifting gantries.  Only in remote areas , where the mobilisation/demobilisation of mobile cranes is very costly, the previously mentioned disadvantages of the lifting gantries still justify the use of such systems.  Hire rates and mob./demob cost of mast gantry systems are in general lower then those of large cranes.   In Western Europe  nearly all large columns are erected by cranes and only in those cases, where cranes cannot offer sufficient lifting capacity, the mast gantry systems offer a good alternative.

Overload Danger!
In this article we will limit ourselves to the erection of a 520 tons pressure vessel by  means of two main lift cranes and one tailcrane. The main consideration which we have to observe is : How do we monitor the load in each main lift crane “A” and “B” during the erection procedure.

As each main lift crane is connected directly to the main lifting trunion (See Fig. 2 below), and not through an equalizing spreader beam, the danger exists that one of the cranes will be overloaded, when they are not lifting the column at the same speed. The answer to this important point is rather simple: We have to use a device which accurately measures the position of each lifting trunion in relation to the horizon during the entire erection procedure, so that we can adjust the the lifting speed of the main lift cranes accordingly. When both lifting trunions are kept in a horizontal plain during the lift, than there will be no overload in any of the two main lift cranes. When crane “A” lifts faster then crane “B”, it will result in an overload of crane “A”. Depending on the distance of the main lifting trunion to the CoG, this overload could  be insignificant or could be dangerous. It is therefore required that these conditions are calculated before one starts the lift and that we keep both main lifting trunions in a horizontal plain during the lift. Let ‘s  make some calculations on the overload on “A’ or “B” when both trunions are not kept horizontal.

Suppose that  our heavy lift pressure vessel is not lifted correctly and that crane “A” lifts faster than crane “B” resulting in a 1o degree angle with the horizon.   Due to this incline with the horizon the load in crane “A” will be : 270.09 Ton  and in crane “B”:  249.91 Ton.  This overload is calculated when the column is supported by both cranes in vertical position. It is  proportional less when the column has not reached it’s vertical position  yet.

The larger the distance between the CoG. and the lifting trunions, the greater the influence of an incline on loads in crane “A’ and “B” will be.  These loads are calculated and as tabulated below:

When the distance of the main lifting trunions is increased to twice the original distance, the overload   will increase to twice the magnitude as well. See table below with a distance of the main lifting trunions increased to 12 m.

What have we learned from this exercise?
First of all , let’s try to install the main lifting trunions as close as possible to the CoG of the pressure vessel. Be aware of the fact that we  should always keep the main lifting trunions at least 2 m above the CoG. and not any closer.  The reason for this is that the main lifting slings are placed around the circumferance of the trunions and the friction  of the slings in the trunions could obstruct the trunions to rotate in the slings, when the column is lifted from horizontal into vertical position.  It will eventually turn, but at the end of the operation, one will find out that the column will not  completely turn into vertical position, and that will hinder the lowering down over the foundation bolts.

In this respect  I would recommend following tips:

  • Place a  2 mm thick steel strip between the trunions and the lifting slings and grease it. This will facilitate the slings to turn better around the trunions surface and at the same time it will protect the sling from wear and tear. See  Fig.4
  • Place  steel guides on  at least 3 foundation bolts.

These guides will ease the lowering down of the columns base ring over the foundation bolts. It is also advisable to connect 4 chain tackles or tirfors  to the base ring during the lowering down procedure. These tools will  be of great help in guiding the base ring and it will  avoid possible damage on the thread of the foundation bolts during lowering down.

We still have not yet discussed the main issue of this operation and that is how do we accurately monitor  that each lifting trunion stays in a horizontal plain during the entire lifting procedure. During a large number of tandem lifts of big columns, I have succesfully used the same method over and over again.

During my first tandem lift back in 1976, I used two methods to monitor possible overload of each crane. Load indicators, which were installed in the dead end of the lifting tackle and an electronic inclino meter, which measured the angle of the imaginary line between both lifting trunions and the horizon (See Figure 4). 

 

I must admit that the load indicators were of no use at all. Due to the friction of the sheaves of both lifting tackles, the variations in the dead end of the tackle of each crane were in no way a reliable  measure  to monitor the operation.  The inclino meter, which we installed on a swivelplate at the basering of the column was far more reliable and prooved to be a very good tool.  The inclinometer is fitted on a multiplex  board with a swivel and can be clamped to the top of the basering or at the bottom. See Figure 5 and picture V52-5.

 

As an extra precaution we attached a normal spirit level on the same swivel plate, so that even in case of an electronic failure we can still monitor the operation.  As can be seen from above tables, it is very important to keep both lifting trunions horizontal during the entire lifting procedure and thus avoid any overloading of any of the main lift cranes.  In normal circumstances, we would position both main lift cranes  and tailcrane as shown in Fig. 2. 

By calculating the decrease of the tailload in relation to the angle with the vertical we can establish the most economical position of  tailcrane “C”. Let ‘s have a look at the values of decrease of the tailload in relation to the angle with the vertical.

With the results of this table we can now establish the optimal position of the tailcrane.

The tailcrane should at least have a capacity of approx. 80 Tons at  approx. 14 m radius. A Liebherr LTM-1400 with Spanlift system has a capacity of 87 tons  at  14 m radius with 36.5 m boomlength.  It seems that this telescopic crane would have sufficient  capacity to handle the tailload of our column. We now have to position the Liebherr at the side of the column between the taillug and the main lift cranes, in such a way that we can really prove that the crane can guide the tailpoint of the column , until it has reached it’s vertical position.

In Fig. 4 the position of each crane is clearly identified.  As main lift cranes we have selected two mobile Demag TC-3000 cranes with Superlift attachment. Each crane is positioned at 10 m radius with 54 m main boom and has a rated lifting capacity of 340 tons with 100 tons Superlift counterweight. The ideal position is that each crane is placed perpendicular to the columns longitudinal centerline.  Both cranes can though be placed under an angle provided the clearance of the boomheads are still garanteed.  In our case, both lifting trunions protrude 650 mm outside the column’s shell, which just gives a theoretical clearance of approx. 150 mm (this depends on the physical dimensions of the crane  boomheads).

The rated lifting capacity  for both cranes in tandem is 2×340 = 680 Tons.  With a weight of 520 tons , this lifting capacity is 520/680 = 76 % of the max. allowable lifting capacity, which we considered safe, provided the precautions are taken as described in this article.  We could even  accept a smaller safety marging, which means that these crane types could even erect columns   close to the 600 tons weight category.

Erection procedure
How do we now control the erection of this column?  In the Netherlands, the usual practice is that such an operation is carried out under supervision of the Rigging Supervisor, who clearly instructs the operators of all three cranes. To limit the amount of instructions given and to make the operation rather simple, a strict procedure is worked out.  Before the cranes are attached to the lifting trunions, the operators and supervisor check the speed of each crane and agree with each other at which speed will be lifted. Both main lift cranes are then attached to the lifting trunions, as well as the tailcrane. Then the column is lifted out of the transport saddles, so that we can adjust the level of both main lifting trunions to an exact horizontal plain. This can easily be done by means of a level instrument. When both lifting trunions are exactly horizontal, then we set the inclino meter and spirit level at the basering of the column to zero degrees as well (Horizontal level). See Figure 5.

The inclinometer and spiritlevel are attached to the basering and will remain in the same vertical plain during the erection from horizontal into vertical position, because of the swivel construction of the multiplex board.  When  the measuring system is set and checked, the  Rigging supervisor instructs  both main lift crane operators to start lifting. The tailcrane is placed in a free swing and gently has to keep the basering from the ground  and  at the same time slowly slews towards  cranes “A” and “B”. One crane i.e. crane “A” is lifting at the same lifting speed during the entire operation.  Adjustments are made by stopping or increasing the speed of crane “B” , while crane “A” continues lifting at the same speed. In this way , the rigging supervisor only has to instruct  crane operator  “B”.  When the column has reached an angle of approx. 70 degrees with the horizontal, the basering is lifted high enough above the ground and final adjustments are made to ensure the horizontallity of the main lifting trunions, so that in one swing without lifting the column any higher, the tailcrane can slowly guide the column into vertical position. This method is followed, to avoid any deviation from the horizontal plain of both main lifting trunions, as this will have max. affect  on a possible overload of crane “A” or “B”.  The most critical part of the operation is when the column has to be swung in between both crane boomheads. It is obvious that there should not be any clips, nozzles or other obstructions in the area along the columns shell , where lifting tackle and boomheads  pass.

The  picture  shows the erection of two large columns at the Fina Refinery in Antwerp in Sept. 1989. Each column weighed 513 Tons and one had a lenght of 76 m , while the other measured 82 m in length, with diameters of 4.5 m and 5.4 m respectively. Originally it was intended to use a Liebherr LTM-1400 as tailcrane and 2x Demag TC-3000 truckcranes as main lift cranes. At the last moment the Liebherr was exchanged by a Demag CC-2000, as the Liebherr was  not available when the lift was planned. The  erection of both columns was executed in a workperiod of only 10 days, this time included the time for mobilisation and rigging of all 3 cranes, lifting the first column, the relocation of one of the TC-3000 cranes (the other TC-3000 could stay in it’s original position) , lifting the second column and the derigging and demobilisation of all cranes from the jobsite.

It speaks for itself, that the crane types used in this article are just examples and that the same  principles can be applied for other crane types.                             _______________